Television receiver with delayed display

ABSTRACT

In a color television receiver including a color kinescope and plural kinescope driver stages, a control circuit is included for inhibiting the operation of the driver stages for a short interval after the receiver is energized, particularly under &#34;hot start&#34; conditions, to prevent disturbing artifacts from being displayed by the kinescope. In an illustrated embodiment the driver stages are disabled in response to a control signal applied to a bias point common to the plural driver stages.

This invention concerns a video signal processing and display system,such as a television receiver, wherein the display of video informationis inhibited for a given time interval after the receiver is energized.

It has been observed that some color television receivers may displayartifacts which are disturbing to a viewer, particularly during a "hotstart" when the receiver is re-energized shortly after having beenturned off. A "hot start" is a condition where the receiver is turned onbefore the filaments of an image displaying kinescope have had a chanceto cool off, e.g., within about two minutes.

The disturbing artifacts displayed after a "hot start" can assume manyforms. In one case a kinescope was seen to display a bright blue field,followed by a bright red field, before the system settled down to anormal display condition, i.e., normal video information or a blackfield in the absence of video information. Some receivers delay thevertical scanning of the kinescope for a few seconds after the receiveris energized to improve kinescope degaussing by eliminating interactionbetween the vertical deflection field and the degaussing coil. In thiscase the disturbing displayed artifacts which appear after a hot startappear, for example, as intense red then blue lines followed afterwardsby an intense white line. In this case a line rather than a full screenfield display occurs because inhibited vertical scanning results in avertical field compressed to a horizontal line with magnified intensity.In addition to being objectionable to a viewer, the described intenselines may produce kinescope screen burn if the automatic kinescope beamcurrent limiter network of the receiver is unable to limit the beamcurrents associated with such displayed intense lines.

The described disturbing artifacts can be attributable to a variety ofcauses. In one case, for example the cause has been traced to theoperation of plural clamping circuits respectively associated with red,green and blue video signal processing circuits preceding the displaydevice. Each such clamp may include keyed comparators with associatedstorage capacitors for establishing a desired DC bias condition for thecolor signal processing path it acts upon. However, the color signalclamps may operate erratically due to improper biasing during the shortinterval after the receiver is energized, i.e., before the receiverpower supplies have stabilized. Erratic clamp operation before properbiasing is achieved, coupled with circuit tolerances and the dischargingcharacteristics of the clamp storage capacitors, appear to beresponsible for the disturbing display artifacts mentioned above.

Accordingly, there is disclosed herein apparatus for preventing thedescribed disturbing artifacts from being displayed. In accordance withthe principles of the present invention, this is accomplished bydecoupling the kinescope signal inputs for display purposes for a givenshort time period when the receiver is energized, particularly under"hot start" conditions.

In a disclosed preferred embodiment of the invention, the signal inputsof a color kinescope are driven by a cascode display driver amplifiercomprising plural signal input common emitter amplifier transistors, andrespectively associated plural signal output common base amplifiertransistors each having a bias input coupled to a common source of biaspotential. The normal biasing of the signal output transistors isdisrupted for a given short time interval after the receiver isenergized, thereby preventing the output transistors from conveyingvideo signals to the kinescope.

The single Figure of the drawing depicts a portion of a color televisionreceiver including display control apparatus in accordance with theprinciples of the present invention.

Television signal processing circuits 10 provide separated luminance (Y)and chrominance (C) components of a composite color television signal toluminance-chrominance signal processing and clamping circuits 12.Processor 12 includes luminance and chrominance gain control circuits,DC level setting circuits, color demodulators for developing r-y, g-yand b-y color difference signals, matrix amplifiers for combining thelatter signals with processed luminance signals to provide low levelcolor image representative output signals r, g and b, and pluralclamping circuits, e.g., clamping comparator circuits of the typedescribed in U.S. Pat. No. 4,554,588--R. L. Shanley for establishing adesired DC condition for each of the r, g and b signals. Clampingcomparator circuits of this type are employed in the CTC-131 colortelevision receiver chassis manufactured by RCA Corporation,specifically in a luminance-chrominance signal processing integratedcircuit as shown in the Basic Service Data publication (1984, secondedition, CTC-131) for this receiver. The r, g and b signals arerespectively amplified by red, green and blue video output kinescopedisplay driver amplifiers 14a, 14b and 14c of similar configuration asthat shown for red driver 14a. Drivers 14a, 14b and 14c provide highlevel amplified color image signals R, G and B to respective cathodeintensity control electrodes 16a, 16b and 16c of a color kinescope 15.In this instance kinescope 15 is of the self-converging, "in-line" guntype with a commonly energized G1 grid electrode associated with each ofthe kinescope electron guns comprising cathode electrodes 16a, 16b and16c.

Driver 14a includes a common emitter input amplifier transistor 20 whichreceives input signal r via a resistor 21, and a high voltage commonbase output amplifier transistor 22 which forms a cascode video outputdisplay driver amplifier stage with input transistor 20. High levelvideo signal R suitable for driving kinescope cathode 16a is developedacross a load resistor 24 in the collector output circuit of transistor22. A high operating voltage for driver 20, 22 is provided by a sourceof positive DC potential B+ (e.g., +230 volts) coupled to the collectorcircuit of transistor 22. Direct current negative feedback is providedfrom the collector output of transistor 22 to the base input oftransistor 20 by means of a feedback resistor 25. Normal bias for thebase electrode of output transistor 22 is provided by a bias voltagesource comprising voltage divider resistors 26 and 27 coupled between asource of operating potential (+11.2 v) and ground. A bias voltage VBdeveloped at the junction of resistors 26 and 27 is coupled in common toeach of the base electrodes of the output transistors of driver stages14a, 14b and 14c.

Automatic kinescope bias (AKB) control networks 13a, 13b and 13c arerespectively associated with each of driver stages 14a, 14b and 14c. Theautomatic bias control networks exhibit similar structure and operatingcharacteristics and serve to maintain a desired black level DC bias forthe respective cathodes of kinescope 15. A sensing resistor 30 in serieswith driver transistors 20, 22 acts in conjunction with the AKB systemby developing a voltage at a sensing node A representative of thekinescope cathode black current level conducted during image blankingintervals in response to a positive grid drive pulse VG applied to gridelectrode G1 during prescribed portions of vertical blanking intervalsas explained, for example, in U.S. Pat. Nos. 4,263,622 and 4,277,798,both of Werner Hinn.

Briefly, during each AKB interval, positive pulse VG forward biases gridG1, thereby causing the electron gun comprising cathode 16a and grid G1to increase conduction. In response to grid pulse VG, a similarlyphased, positive current pulse appears at cathode 16a during the gridpulse interval. The amplitude of the cathode output current pulse isproportional to the level of cathode black current conduction (typicallya few microamperes). The induced positive cathode output pulse appearsat the collector of transistor 22. This pulse is fed back to the baseinput of transistor 20 through resistor 25, causing the currentconduction of transistor 20 to increase proportionally while the cathodepulse is present. The increased current conducted by transistor 20causes a voltage to be developed across sensing resistor 30. Thisvoltage is in the form of a negative-going voltage pulse which appearsat sensing node A and which is proportional in magnitude to themagnitude of the black level representative cathode output pulse.

The recovered black current representative voltage pulse is coupled fromnode A via an AC coupling capacitor 34 to a sampling and control signalprocessing circuits in bias control network 13a. Keyed sample and holdcircuits within network 13a are enabled by a sampling timing signal fordeveloping a DC bias control voltage proportional to the magnitude ofthe voltage pulse developed at node A. The bias control voltage isstored and is applied via a resistor 38 to a bias control input at thebase of transistor 20 for maintaining a desired cathode bias voltagecorresponding to a desired black level cathode current. Illustratively,if the magnitude of the induced cathode output pulse corresponds to acondition of excessive black current the bias control voltage decreasesto thereby increase the bias voltage of cathode 16a at the collector oftransistor 22. This reduces the black current level to the correctlevel. Networks 13a, 13b and 13c can employ signal sample and holdnetworks of the type described in U.S. Pat. No. 4,331,981 and U.S. Pat.No. 4,331,982, both of R. P. Parker, and can also employ sampling andcontrol voltage processing circuits of the type shown in U.S. Pat. No.4,277,798 of Werner Hinn.

The television receiver is energized in response to plural receiveroperating supply voltages provided from a source 44 when source 44 isenergized from a source of AC power 40 when a viewer operated powerswitch 42 is placed in the "ON" position. The operating voltages fromsource 44 include supply voltages for signal processing circuits of thereceiver as well as operating voltages for kinescope 15 (e.g., includingfilament heater and very high anode voltages). In particular, source 44provides an operating voltage VCC of +11.2 volts for various signalprocessing circuits and from which the bias voltage for the baseelectrodes of the common base output transistors (e.g., transistor 22)is derived.

In accordance with the principles of the present invention, a timingcircuit 50 serves to modify the base bias voltage of the outputtransistors when the receiver is energized, as will be described below.Circuit 50 includes a switching transistor 52 with a collector outputcoupled to the junction of bias supply resistors 26 and 27 and to thebase electrodes of the common base output transistors of each displaydriver stage. The base input electrode of transistor 52 is coupled tobias potential VCC via a series capacitor 54 and a resistor networkincluding a series resistor 55 and a shunt resistor 56. A normallynonconductive diode 58 is coupled from the negative (-) terminal ofcapacitor 54 to a point of reference potential (ground).

After the receiver has been operating for a while in a steady statecondition, capacitor 5 is charged to 11.2 volts. At this time transistor52 is non-conductive and the display driver stages operate normally toprovide video signals to the kinescope. When the receiver is then turnedoff via switch 42, the level of voltage VCC from source 44 decreasesrapidly. Diode 58 conducts, and the negative terminal of capacitor 54 isclamped to a voltage of -0.7 volts, i.e., one diode offset voltage dropbelow ground reference potential. Capacitor 54 is discharged rapidly viaa discharge path including the positive terminal of capacitor 54, a loadimpedance (not shown) exhibited between the VCC voltage terminal ofsource 44 and ground, the anode of diode 58 and the negative terminal ofcapacitor 54. With the receiver off, a voltage of approximately 0.7volts appears across capacitor 54.

The receiver may be re-energized a short time, e.g. within one or twominutes, after being turned off. This corresponds to a "hot start"condition wherein the kinescope cathodes remain hot or warm and arecapable of immediate emission such that the kinescope can produce animage display immediately. The VCC operating potential appears quickly.The positive and negative terminals of capacitor 54 then exhibitvoltages of +11.2 volts and +10.5 volts, respectively, and diode 58 isnonconductive. The +10.5 volt potential at the negative terminal ofcapacitor 54 is conveyed via voltage divider resistors 55 and 56 to thebase of transistor 52, causing it to conduct in a saturated statewherein the collector potential of transistor 52 (a few tenths of avolt) closely approximates its emitter potential of zero volts.Accordingly, the bias voltage at the junction of resistors 26 and 27decreases and the base bias applied to the base electrodes of the videooutput transistors is now determined by the low collector potential oftransistor 55, which is insufficient to maintain the video outputtransistors forward biased in a conductive state. The output transistorsare thereby rendered nonconductive and the outputs of video signalprocessing circuits 12 are therefore decoupled from the kinescopecathodes. Any disturbing artifacts which would otherwise be developed asa consequence of a "hot start" condition are prevented from beingdisplayed as long as transistor 52 is sufficiently conductive to preventnormal bias voltage VB from being developed at the base electrodes ofthe video output transistors.

Capacitor 54 eventually charges to +11.2 volts in accordance with a timeconstant determined by the values of resistors 55 and 56 and the valueof capacitor 54. At such time, e.g., a few seconds after the "hot start"energization of the receiver, transistor 52 is rendered nonconductiveand normal biasing of the base electrodes of the video outputtransistors returns, whereby these transistors are rendered conductiveand the outputs from signal processing circuits 12 are coupled to thekinescope cathodes.

The described timing circuit is advantageously used with plural cascodedisplay driver stages since in such case a common bias point isavailable at the base electrodes of the common base video outputtransistors for controlling the conductive state thereof via circuit 50.That is, only a single timing circuit 50 is needed. In addition, theconnection of control circuit 50 to the base electrodes of the commonbase output transistors advantageously does not interfere with thesignal processing characteristics of the display drivers, particularlywith respect to the high frequency response thereof.

Due to the manner in which circuit 50 is connected, circuit 50 controlsthe conductive state of the display driver stages under both hot startconditions as described, and also under cold start conditions whereinthe kinescope filaments and cathodes have had sufficient time to coolafter the receiver has been turned off. However, the operation ofcircuit 50 under cold start conditions is not detrimental to theoperation of the receiver. In this regard it is noted that the operationof the AKB system is allowed to begin immediately for both hot and coldstart conditions. In some systems, for cold start conditions anexcessively bright image may be produced initially due to AKB systemaction if the AKB system is enabled to operate immediately after a coldstart, for reasons explained in U.S. Pat. No. 4,450,476--J. C. Tallant,II. To eliminate such excessively bright initial image display, theoperation of the AKB system can be delayed after a cold start, asdescribed in U.S. Pat. No. 4,450,476, by the use of an AKB "hold-off"circuit. It has been found that with the use of circuit 50 of thepresent disclosure the use of such an AKB "hold off" circuit can bedispensed with since after a cold start the display driver stages areenabled to conduct after a delay determined by the time constant ofcircuit 50. This delay in the conduction of the display driver stagesprevents an excessively bright initial image display which in somesystems would necessitate the use of an AKB hold-off circuit of the typedescribed in the Tallant patent.

What is claimed is:
 1. A video signal processing and display systemcomprising:an image display device for displaying an image in responseto an image representative video signal applied thereto; means forapplying said video signal to said display device; means for supplyingsystem operating voltages when said system is energized; control meansresponsive to the energization of said system for generating a controlsignal for a prescribed interval upon energization of said system; andmeans for coupling said control signal to said applying means forpreventing normal application of said video signal to said displaydevice by said applying means to prevent said display device fromdisplaying disturbing visual artifacts after a hot-start energization ofsaid system.
 2. A system according to claim 1 whereinsaid applying meansis a display driver amplifier stage.
 3. A system according to claim 2,whereinsaid display driver amplifier stage comprises a transistor with asignal input, a signal output and a bias input; and said control signalis applied to said bias input for modifying normal biasing of saidtransistor so as to inhibit normal operation of said transistor.
 4. Asystem according to claim 2, whereinsaid display driver amplifier stageis a cascode amplifier having a signal input amplifier device, and asignal output amplifier device with a bias input; and said controlsignal is applied to said bias input for modifying normal biasing ofsaid signal output amplifier device so as to render said signal outputamplifier device nonconductive.
 5. A system according to claim 2,whereinsaid image display device is a color image display device havingplural signal inputs; said display driver amplifier stage comprisesplural cascode amplifier stages respectively associated with said pluralsignal inputs of said display device, each said cascode amplifier stagecomprising a signal input amplifier device, and a signal outputamplifier device having a bias input; and said control signal is coupledin common to said bias inputs of said signal output amplifier devicesfor modifying normal biasing of said plural signal output amplifierdevices so as to render said plural signal output devices nonconductiveduring said prescribed interval.
 6. A system according to claim 5,whereineach of said signal output amplifier devices is a transistorhaving input and output electrodes defining a main current conductionpath of said signal output amplifier device, and a bias input electrode.7. A system according to claim 1, whereinsaid image display device is akinescope having a cathode signal input electrode for receiving saidimage representative video signal; and said system includes bias controlmeans coupled to said kinescope for automatically controlling the biasof said kinescope.
 8. A system according to claim 1, wherein saidcontrol means comprises:a switching transistor having an outputelectrode coupled to said applying means, and an input electrode; acapacitor having a first terminal coupled to an operating voltagedeveloped by said supplying means, and a second terminal; a resistivenetwork for coupling said second terminal of said capacitor to saidinput electrode of said switching transistor; and a clamping diodecoupled to said capacitor.